hydraulic
and pneumatic fluids and componentsof
fluid power systems. While having a knowledgeof
system components is essential, it is difficultto
understand the interrelationship of thesecomponents
by simply watching the systemoperate.
The knowledge of system interrelationis
required to effectively troubleshoot andmaintain
a fluid power system. Diagrams pro-videdin
applicable technical publications ordrawings
are a valuable aid in understanding theoperation
of the system and in diagnosing thecauses
of malfunctions.

This chapter explains the different types of

diagrams
used to illustrate fluid power circuits,including
some of the symbols that depict fluidpower
components. Included in this chapterare
descriptions and illustrations denoting thedifferences
between open-center and closed-centerfluid
power systems. The last part of the chapterdescribes
and illustrates some applications of basicfluid
power systems.

DIAGRAMS

As mentioned earlier in this chapter, to

troubleshoot
fluid power systems intelligently, amechanic
or technician must be familiar with thesystem
on which he or she is working. Themechanic
must know the function of eachcomponent
in the system and have a mentalpicture
of its location in relation to othercomponents.
This can best be done by studyingthe
diagrams of the system.

A diagram may be defined as a graphic

representation
of an assembly or system thatindicates
the various parts and expresses themethods
or principles of operations. The abilityto
read diagrams is a basic requirement forunderstanding
the operation of fluid powersystems.
Understanding the diagrams of a systemrequires
having a knowledge of the symbols usedin
the schematic diagrams.

SYMBOLS

The Navy uses two military standards that

list
mechanical symbols that must be used inpreparing
drawings that will contain symbolicrepresentation.
These standards are as follows:1. Military
Standard, Mechanical Symbols(Other
than Aeronautical, Aerospacecraft, andSpacecraft
Use), Part 1, MIL-STD-17B-1.2. Military
Standard, Mechanical Symbols forAeronautical,
Aerospacecraft, and SpacecraftUse,
Part 2, MIL-STD-17B-2.

are
not all encompassing, they do provide a basisfor
an individual working with fluid powersystems
to build upon. Some rules applicable tographical
symbols for fluid diagrams are asfollows:

1. Symbols show connections, flow paths,

and
the function of the component representedonly.
They do not indicate conditions occurringduring
transition from one flow path to another;nor
do they indicate component construction orvalues,
such as pressure or flow rate.

2. Symbols do not indicate the location of

ports,
direction of shifting of spools, or positionof
control elements on actual components.

3. Symbols may be rotated or reversed

without
altering their meaning except in cases oflines
to reservoirs and vented manifolds.

4. Symbols may be drawn in any size.

5. Each symbol is drawn to show the normal

or
neutral condition of each component unlessmultiple
circuit diagrams are furnished showingvarious
phases of circuit operation.

For more detailed information concerning the symbols
used in fluid power diagrams, consult the above-mentioned
military standards. Additional
information
concerning symbols and the reading

of
diagrams is contained in BIueprint
Reading andSketching, NAVEDTRA
10077-F1.

TYPES OF DIAGRAMS

There are many types of diagrams. Those that

are
most pertinent to fluid power systems arediscussed
in this text.

Pictorial Diagrams

Pictorial diagrams (fig. 12-1) show the

general
location and actual appearance of eachcomponent,
all interconnecting piping, and thegeneral
piping arrangement. This type of diagramis
sometimes referred to as an installationdiagram.
Diagrams of this type are invaluable tomaintenance
personnel in identifying and locatingcomponents
of a system.

Cutaway Diagrams

Cutaway diagrams (fig. 12-2) show the internal

working
parts of all fluid power components ina
system. This includes controls and actuatingmechanisms
and all interconnecting piping.Cutaway
diagrams do not normally use symbols.